I just started on spiro 50mg about 2 weeks ago ( I also have moderate-severe cystic acne), and my derm told me it would take about 3 months to see results, with full results after about 6 months. However, I was only supposed to be on the 50mg for the first 10 days and then bump up to 100mg. I was having some troubling side effects (severe headaches, breathing difficulties, low blood pressure) and I think it would be really tough on my system to double the dose, so I'm staying at 50mg. From what I've seen on these forums, most people recommend 100-200mg to really see drastic results after the first few months and then lowering down to a maintenance dose of 50-100mg. If you haven't been having any side effects on the 50mg, I would ask you're derm about upping your dose to 75-100. The 50 is fine, but if you can handle more, you might see better results in the long run. Also, I've seen that adding a birth control regimen (usually Yaz or Dianette) increases the skin-clearing effects of the spiro. Personally, I'm not using a birth control, because I want to see how well the spiro works by itself.
Its a lengthy process, but I've dealt with acne for 10 years, so a few months seems like no time if the spiro actually helps things in the end